When do we know a language? In English, when we say a penny for your thoughts, we are using an idiom. We can express ourselves more succinctly and natively using these. Beating around the bush is another one. If we pick up enough of these and hurl them around at times, this will makes us fluent.
It is almost the same with programming languages. You see a construct time and again and make use of notable features of a specific programming language. Here is a sample of idioms from a few prominent languages.
For example, here is an idiomatic Scala way to sum up two lists of numbers:
scala> val d1 = List(1, 2, 3, 4, 5) d1: List[Int] = List(1, 2, 3, 4, 5) scala> val d2 = List(11, 22, 33, 44, 55) d2: List[Int] = List(11, 22, 33, 44, 55)